Growth history and ecology of the Atlantic surf clam,Spisula solidissima (Dillwyn), as revealed by stable isotopes and annual shell increments

The shells of the Atlantic surf clam, Spisula solidissima (Dillwyn), contain a record of both life history and environmental changes. These shell records were investigated using oxygen and carbon stable isotopic analyses (δ¹⁸O, δ¹³C) and shell growth increment analyses. δ¹⁸O variations across annual shell increments reflect the yearly cycle of sea-water temperatures off the New Jersey coast, further documenting the proposed annual periodicity of the major shell increments. The 11-yr shell record analyzed here confirms that shell growth is most rapid in spring-early summer, slow in late summer-fall, and extremely slow or non-existent in winter. Shell growth appears to occur in isotopic equilibrium with sea water and measured δ¹⁸O values are used to refine the aragonite-water temperature scale. Variations in the timing of annual growth increment formation are noted as well as ontogenetic effects upon the range of isotopic values recorded in shell carbonate. Both the δ¹⁸O and δ¹³C profiles are influenced by changes in the sea-water temperature regime over the 11-yr period studied (1965–1976) and record these effects in the shell. The combination of stable isotope and growth increment analyses provides a powerful tool for interpreting the shell records of both modern and fossil molluscs.